In modern communications systems using CDMA or OFDM modulation schemes, the signal is not a constant envelope signal, which means higher linearity requirements for the power amplifier (PA). In a wideband code division multiple access (WCDMA) system, the downlink signal is a sum of signals intended for different users. If the independent constituent carriers are combined in a multicarrier system in the digital domain, the Peak-to-Average Power Ratio (PAPR) grows even further. OFDM systems are multicarrier by definition, therefore leading to high PAPR as well. In all the above cases, the composite signal is Gaussian distributed that leads to a high peak-to-average-power-ratio (PAPR). High linearity requirements lead to low power efficiency and, therefore, to high power consumption in the PA. Nevertheless, for cost efficient implementation, it is still beneficial to combine the carriers in the digital intermediate frequency (IF) domain. The PAPR of the combined signal must be reduced in order to achieve a good efficiency in the PA, implying that the signal amplitude must be limited to have lower peaks, e.g. by clipping the signal. Advanced clipping methods were recently developed. One of them, the peak windowing method, can be applied on GSM, WCDMA, and OFDM single and multicarrier transmissions.
The simplest way to reduce the PAPR is to literally clip the signal, but this significantly increases the out-of-band emissions. A different approach is to multiply large signal peaks with a certain window. Any window can be used, provided it has good spectral properties. Examples of suitable window functions are the Gaussian, cosine, Kaiser, Hanning, and Hamming windows.
The function of the ‘simple’ (not windowed) peak suppression can be expressed as follows:sc(n)=s(n)c(n)where s(n) is the original signal sample, sc(n) is the clipped sample, c(n) is the clipping factor obeying
                              c          ⁡                      (            n            )                          =                  {                                                                      1                  ,                                                                                                                                            s                      ⁡                                              (                        n                        )                                                                                                  ≤                  A                                                                                                                          A                                                                                        s                        ⁡                                                  (                          n                          )                                                                                                                            ,                                                                                                                                            s                      ⁡                                              (                        n                        )                                                                                                  >                  A                                                                                        (        1        )            
with A being the maximum allowable amplitude of the signal.
Although (1) preserves the signal phase, abrupt amplitude changes appear in the clipped signal, causing significant out-of-band spectrum re-growth. To overcome this drawback, a somewhat graceful clipping of the signal amplitude is suggested by the peak-windowing approach, by replacing the clipping series c(n) with a new one, b(n), computed by:
                              b          ⁡                      (            n            )                          =                  1          -                                    ∑                              k                =                                  n                  -                  N                  +                  1                                            n                        ⁢                                          [                                  1                  -                                      c                    ⁡                                          (                      k                      )                                                                      ]                            ⁢                              w                ⁡                                  (                                      n                    -                    k                                    )                                                                                        (        2        )            
where w(n) is the chosen window function truncated to length N.
In practice, however, the peaks may often appear rather frequently in the signal time envelope; this leads to overlapping of windows surrounding closely located peaks and, therefore, to excessive clipping of the signal and, thus, to its unreasonable distortion.
One suggested remedy to this drawback is adding feedback to the circuit implementing (2), in order to predict and prevent unnecessary clipping. FIG. 1 is a diagram of such an improved circuit available in the prior art, utilizing windowing as Finite Impulse Response (FIR) filter and a protective feedback. While providing the desired improvement, the feedback portion breaks the simple structure of the FIR filter, which is well exploited in the industry, and generally is less suitable for a pipelined design supporting high sampling rates of the data.